Python f.write() at beginning of file?

Question

I'm doing it like this now, but i want it to write at the beginning of the file instead.

f = open('out.txt', 'a') # or 'w'?
f.write("string 1")
f.write("string 2")
f.write("string 3")
f.close()

so that the contenst of out.txt will be:

string 3
string 2
string 1

and not (like this code does):

string 1
string 2
string 3

Answer 1

You could throw a f.seek(0) between each write (or write a wrapper function that does it for you), but there's no simple built in way of doing this.

EDIT: this doesn't work, even if you put a f.flush() in there it will continually overwrite. You may just have to queue up the writes and reverse the order yourself.

So instead of

f.write("string 1")
f.write("string 2")
f.write("string 3")

Maybe do something like:

writeList = []
writeList.append("string 1\n")
writeList.append("string 2\n")
writeList.append("string 3\n")
writeList.reverse()
f.writelines(writeList)

Answer 2

Elaborating on Daniel DiPaolo's answer:

Simply append all the lines that you want to write to a list. Reverse the list and then write its contents into the file.

f=open('output.txt','w')

l=[]
l.append("string 1")
l.append("string 2")
l.append("string 3")

for line in l:
    f.write(line)

f.close()

You could also use a deque and add lines at its beginning instead of using a list and reversing it.

Answer 3

Take a look at this question. There are some solutions there.

Though I would probably go that same way Daniel and MAK suggest -- maybe make a lil' class to make things a little more flexible and explicit:

class front_appender:
    def __init__(self, fname, mode='w'):
        self.__f = open(fname, mode)
        self.__write_queue = []

    def write(self, s):
        self.__write_queue.insert(0, s)

    def close(self):
        self.__f.writelines(self.__write_queue)
        self.__f.close()

f = front_appender('test_d.out')
f.write('string 1\n')
f.write('string 2\n')
f.write('string 3\n')
f.close()